1. Field of the Invention
The present invention relates to a technique of etching a substrate having a silicon nitride film and a silicon oxide film exposed on the surface thereof in such a manner that the silicon nitride film is selectively etched.
2. Description of the Related Art
Treatments for manufacturing semiconductor devices include wet etching treatment for etching silicon nitride film formed on the surface of a substrate such as a semiconductor wafer with an etching solution. In wet etching treatment, it is often the case that silicon nitride film is required to be selectively etched substantially without etching silicon oxide film. In order to achieve such etching, a phosphoric acid solution (H3PO4) has been conventionally used as the etching solution (refer to JP-A-2004-221540).
However, usage of phosphoric acid solution requires provision of an exclusive supply system for the phosphoric acid and consideration for preventing the clean room atmosphere from being contaminated by the phosphoric acid, and there are also disadvantages such as the high cost of phosphoric acid with high purity. Alternative etching solutions are therefore recently becoming to be used instead of phosphoric acid.
As an example of such etching solution that does not contain phosphoric acid, JP-A-2010-147304 describes an etching solution containing sulfuric acid, a fluoride compound, and water. It also describes that hydrofluoric acid and ammonium fluoride are preferably used as the fluoride compound in manufacturing of electronic devices such as semiconductors.
However, regardless of whether a substrate is etched by using a phosphoric acid etching solution or hydrofluoric acid etching solution, in either case it is actually difficult to enhance the ratio of the etching amount of the silicon nitride film to the etching amount of the silicon oxide film (hereinafter also referred to as “SiN/SiO2 selectivity”) to a level required to manufacture a semiconductor device.